Interpretive Summary: Deli-type salads have a high incidence of contamination with Listeria monocytogenes. Since deli salads are not cooked before consumption, the presence of L. monocytogenes in deli salads is a health concern to consumers if L. monocytogenes can grow during storage. To better understand its behavior in deli salads, L. monocytogenes was studied in egg salad and pasta salad formulated with various mayonnaise pH (3.8-5.0) and stored at refrigerated temperature (4 degrees C) and abuse temperatures (8 degrees and 12 degrees C). L. monocytogenes was able to grow in both egg salad and pasta salad at all storage temperatures. The growth was significantly slower in salads stored at lower temperatures. This indicates that the growth of L. monocytogenes in salads could be reduced by simply keeping the products at refrigerated temperature. A combination of using mayonnaise of low pH in preparing salads and storing the finished product at refrigerated temperature further reduced the growth of L. monocytogenes. Mathematical models that described the growth of L. monocytogenes in egg salad and pasta salad at storage temperatures of 4 degrees-12 degrees C were developed. The growth data of L. monocytogenes in salads and the models can be used by salad manufacturers, retailers, and consumers to identify product formulations and product handling practices that can reduce the potential growth of L. monocytogenes.

Technical Abstract:
The growth of L. monocytogenes was more rapid in egg salad than in pasta salad, indicating that a better growth condition for L. monocytogenes existed in egg salad. This study investigated and modeled the behavior of L. monocytogenes in egg salad and pasta salad as affected by mayonnaise pH (3.8, 4.2, 4.6, and 5.0) and storage temperatures (4 degrees, 8 degrees, and 12 degrees C). At each storage temperature, L. monocytogenes was able to grow in both salads regardless of the pH of mayonnaise. The lag phase durations (LPD, h) of L. monocytogenes in egg salad were 33-85, 15-50, and 0-19 h, and the growth rates (GR, log 10 cfu/h) were 0.0187-0.0318, 0.0387-0.0512, and 0.0694-0.1003 log 10 cfu/h at 4 degrees, 8 degrees, and 12 degrees, respectively. In both salads, the LPD decreased and the GR increased as the storage temperature increased. Results showed that the storage temperature was the main effector, whereas the mayonnaise pH showed less effect on the growth of L. monocytogenes in both salads. Mathematical models and response surface plots describing the LPD and GR of L. monocytogenes in both salads as a function of the mayonnaise pH and storage temperature were developed. The models showed that the growth of L. monocytogenes in egg salad and pasta salad was primarily promoted by higher storage temperatures and, secondarily by higher mayonnaise pH, indicating that the products should not be stored at elevated storage temperatures. The conditions under which the models may be applied to predict the growth of L. monocytogenes in both salads were identified.